The essential feature of this process consists of generating, by chemical union between oxygen and aluminum, an intense local heat which produces from the reaction a certain amount of molten iron. This iron, which is in the nature of mild steel, may be run into a mould to form a small casting, or may be run between the two parts of a broken forging or steel casting to form a weld. The welding of cast iron by this process is very difficult because the heat of the process burns out the un-combined carbon of the casting, making it hard, brittle, and of uncertain strength.

Aluminum and oxygen have a very strong chemical affinity for each other. A mixture of finely ground aluminum and iron oxide 25 in proper chemical proportion, known by the trade name Thermit, will, when ignited, burn with an intense heat and release molten iron which may be used as stated. The chemical reaction is expressed by Fe2O3 +2Al = Al,O3 + 2Fe. More iron than comes from the oxide may be supplied by placing iron punchings in the mixture before it is ignited, and the heat of the reaction will melt this iron and mix it with that formed from the iron oxide.

Ignition is accomplished by an ignition powder consisting of barium oxide mixed with powdered aluminum. A small quantity of this placed on top of a thermit charge and ignited with a match, will start the burning of the thermit.

This process is advantageously used in welding broken locomotive frames, spokes of heavy cast-steel wheels, ships' stern posts, rudder posts, propeller struts and breaks in other large forgings or steel castings, without removing them from their places. Its heat is also used in welding together lengths of heavy wrought iron or steel pipe. Lengths of street railway rails which are prevented by street paving from subsequent bending under expansion due to the sun's heat are smoothly and economically welded by this method. It is particularly suited to large work and is not economical for small work.